Electronic security systems are known for the detection of the unauthorized removal of items containing a resonant tag circuit. Such systems employ a transmitter providing an electromagnetic field in a zone or region under surveillance, and a receiver operative to detect a resonant tag frequency caused by the presence of a tag in the surveillance zone and to provide an output alarm indication of tag presence. A preferred electronic security system is described in U.S. Pat. No. 3,810,147, 3,863,244 and 3,967,161.
In electronic security such as those described in the above-cited patents, two identical planar single loop antennas are usually employed, one for transmitting and one for receiving. The transmitting loop antenna generates an electromagnetic field which extends far beyond the immediate area of the security system necessary for system operation. In addition, the receiving antenna is sensitive to external noise generated at great distances from the receiver relative to the small area of interest to system operation.
An antenna system is described in U.S. Pat. No. 4,016,553 in which the inherent problems of a simple loop antenna in an electronic security system are minimized by use of two or more identical parallel loop antennas connected in phase opposition or bucking relationship. The antenna system comprises a cluster of at least two parallel electrically conductive loops of similar size connected in phase opposition so that current always flows in mutually opposite directions through corresponding portions of each loop. As a result, the loops are magnetically arranged in a bucking relationship. The length of and spacing between the loops is small compared to the wavelength of the transmitted or received signals and is disclosed to be typically one tenth of the wavelength. The spacing between the parallel loops is an appreciable fraction, for example one fourth, of the width of the egress passage through which a detectable resonant circuit must pass in a security installation. A separate antenna cluster composed of phase opposed parallel loops can be connected to respective transmitter and receiver of the system, or a single antenna cluster can be employed with both the transmitter and receiver. At distances large compared to the dimensions of the transmitting antenna, the generated electromagnetic waves are cancelled by reason of the phaseopposed loop connection. At short distances between the receiving and transmitting antennas, the signals in adjacent parallel antenna conductors do not cancel, resulting in a net detectable signal. Electromagnetic waves incident on the receiving antenna from distances large compared to the antenna dimensions do not provide a sensible antenna signal, but electromagnetic waves incident upon the receiving antenna from sources close to the antenna are sensed to provide a electromagnetic waves incident receiving antenna signal.
Thus the antenna system described in U.S. Pat. No. 4,016,553 provides an electromagnetic field in an interrogation region while preventing high intensity fields from occuring outside of the interrogation region. This antenna system also provides detection of selected electromagnetic fields originating in the interrogation region from a resonant circuit while avoiding detection of fields originating from outside of the interrogation region.
The antenna system described in the aforesaid U.S. Pat. No. 4,016,553 suffers several disadvantages in practice. The bucking loop antennas must be separated by a significant distance relative to the distance between the transmitting antenna cluster and receiving antenna cluster. Moreover, the bucking loop antennas must be carefully aligned and balanced for optimum effect. The loops of an antenna cluster are typically spaced apart from each other by a distance corresponding to one fourth the distance across the egress passage. The size of the antenna cluster can become cumbersome for passage widths of conventiently large dimension. For example, for a passage width of six feet, the antenna cluster must be sufficiently large to accommodate a loop spacing of eighteen inches.
An improved antenna system for use with an electronic security system for the detection of resonant tag circuits is the subject of copending application Ser. No. 878,753, filed Feb. 17, 1978 of the same inventor as herein, and comprises a pair of substantially identical planar multiple loop antennas respectively connected to the transmitter and receiver of the security system and providing an electromagnetic field of high intensity in the interrogation region of the system while preventing high intensity fields at distances outside of the interrogation region which are large in comparison to the antenna dimensions. The antenna system also discriminates against interferring signals originating outside of the interrogation region at distances large compared with the antenna dimensions. Each planar antenna includes two or more loops lying in a common plane, with each loop being twisted 180.degree. with respect to each adjacent loop to be in phase opposition. The transmitting antenna and receiving antenna are symmetrical, that is, identical or nearly so with respect to the number and size of the two or more loops, and are cooperative in that twisted loops of the receiving antenna reverse or decode the adjacent phase relationships of the twisted loops of the transmitting antenna. For each antenna, the total loop area of one phase is equal to the total loop area of opposite phase in order to achieve optimum performance. The antenna system is also effective to provide higher resonant tag detection sensititvity than conventional loop antennas.